This invention relates to the field of laser illumination. More particularly, this invention relates to the field of laser illumination where an intensity detector observes a surface illuminated by the laser illumination and where it is desirable to reduce speckle observed by the intensity detector.
A human eye has finite resolution. When the eye views an object, the eye quantizes the object into resolution spots, each of which are point spread functions of the eye. For example, if a person stands about 3 meters from a surface, the eye resolves the surface into the resolution spots with each of the resolution spots having a diameter of about 1 mm.
FIG. 1 illustrates the eye 12 viewing a diffuse surface 14. A laser illumination 16 illuminates the diffuse surface 14. A particular resolution spot 18 is imaged onto a retina of the eye 12. Features of the diffuse surface 14 that are within the resolution spot 18 are not resolvable by the eye 12. The diffuse surface includes many scattering centers within the resolution spot 18. The scattering centers scatter the laser illumination 16 which is illuminating the resolution spot 18. Because the laser illumination 16 is coherent, the scattering centers create interference within the eye 12. The interference causes the eye 12 to perceive the resolution spot on a brightness scale ranging from a bright spot to a dark spot.
Each scattering center forms a source of lightwaves. The lightwaves constructively interfere; or the lightwaves partially constructively interfere and partially destructively interfere; or the lightwaves destructively interfere. If the lightwaves constructively interfere, the resolution spot 18 is the bright spot. If the lightwaves partially constructively interfere and partially destructively interfere, the resolution spot 18 has an intermediate brightness forming an intermediate brightness spot. If the lightwaves destructively interfere, the resolution spot 18 is the dark spot.
Thus, the eye 12 images the diffuse surface 14 into surface resolution spots in a random pattern of bright spots, intermediate brightness spots, and dark spots. This is speckle. More generally, an optical system which employs an intensity detector will also detect the speckle. One skilled in the art will recognize that the eye 12 is a biological optical system in which the retina functions as the intensity detector. A camera employs a type of intensity detector, which is film for a conventional camera or, typically, a charge coupled device for a digital camera. Thus, a photo of the diffuse surface 14 will show the speckle. FIG. 2 is a photo of speckle 19 which shows a granular pattern of the bright spots, the intermediate brightness spots, and the dark spots.
A measure of the speckle is contrast (C). The contrast, in percent, is given by C=100*IRMS/ where  is a mean intensity and IRMS is a root mean square intensity fluctuation about the mean intensity.
Goodman in xe2x80x9cSome fundamental properties of speckle,xe2x80x9d J. Opt. Soc. A., Vol. 66, No. 11, November 1976, pp 1145-1150, teaches that the speckle can be reduced by superimposing N uncorrelated speckle patterns. This reduces the contrast by a speckle reduction factor of  provided that the N uncorrelated speckle patterns have equal mean intensities and contrasts. If the N uncorrelated speckle patterns have non-equal mean intensities or non-equal contrasts, the speckle reduction factor will be less than . Thus, the speckle reduction factor of  is a best case for the speckle reduction for the N uncorrelated speckle patterns. Goodman further teaches that the uncorrelated speckle patterns can be obtained by means of time, space, frequency, or polarization.
A speckle reduction method of the prior art creates multiple speckle patterns by moving a viewing screen in an oscillatory motion, which employs the time means taught by Goodman. The oscillatory motion typically follows a small circle or a small ellipse about the optic axis. This causes the speckle pattern to shift relative to the eye 12 viewing the viewing screen and, thus, forms multiple speckle patterns over time. Though the amount of the speckle at any instant in time is unchanged, the eye 12 perceives the reduced speckle provided that the speed of the oscillatory motion is above a threshold speed. Stated another way, the eye 12 detects reduced speckle if an integration time for the eye 12 is sufficiently long that the oscillatory motion produces the uncorrelated speckle patterns within the integration time.
In the art of laser illuminated display systems, it is known that an active diffuser can be added to a laser illuminated imaging system to reduce laser speckle. The active diffuser is placed in an intermediary image plane or near the intermediary image plane. The active diffuser is moved in the intermediate image plane in a rotation or toroidal pattern about a display system optic axis in order to create a shifting phase at a display screen. The shifting phase creates uncorrelated speckle patterns over time, thus employing the time means, taught by Goodman.
Wang et al. in xe2x80x9cSpeckle reduction in laser projection systems by diffractive optical elements,xe2x80x9d Applied Optics, Vol. 37, No. 10, April 1998, pp 1770-1775, teach a method of laser speckle reduction in a laser projection system such as a laser television system. In the laser projection system a laser spot forms an image on a display screen by a raster scan similarly to how an electron beam forms an image in a CRT (cathode ray tube) display. The method taught by Wang et al. is accomplished by expanding a laser beam, placing a diffractive optical element in the expanded laser beam to form multiple beamlets, and then focusing the laser beamlets to form the laser spot on the display screen. The multiple beamlets shift slightly as each pixel is formed on the display screen. This provides a time varying speckle pattern and consequently a speckle reduction. Wang et al. further teach that the diffractive optical element can be rotated to slightly improve the speckle reduction.
Bloom et al. in U.S. Pat. No. 5,982,553 issued on Nov. 9, 1999, incorporated herein by reference, teach a display system including a grating light valve, red, green, and blue lasers, various lens arrangements, a scanning mirror, a display screen, and electronics. The electronics control the grating light valve, the lasers, and the scanning mirror to form a two dimensional image on the display screen.
In the display system taught by Bloom et al., the grating light valve forms a line image composed of a linear array of pixels on the display screen. The scanning mirror repeatedly scans the line image across the display screen in a direction perpendicular to the line image as the grating light valve modulates the linear array of pixels thereby forming the two dimensional image.
Because the two dimensional image taught by Bloom et al. is formed by laser illumination, the two dimensional image exhibits laser speckle, which degrades an image quality. It would be desirable to improve the image quality by reducing the laser speckle.
What is needed is a method of reducing laser speckle in a laser illuminated display system where a two dimensional image is formed on a display screen.
What is needed is a method of reducing laser speckle in an optical system where a laser illumination illuminates a diffuse surface.
The present invention is a method of reducing speckle, an apparatus for reducing speckle, a display apparatus featuring reduced speckle, and a diffuser for reducing speckle. The method of the present invention includes dividing a laser illuminated area into phase cells, subdividing the phase cells into a number of cell partitions, and applying a temporal phase variation to the cell partitions within an integration time of an intensity detector viewing the laser illuminated area. If the temporal phase variation is optimally applied, the intensity detector detects an optimum speckle reduction which corresponds to a square root of the number of cell partitions.
In order for the intensity detector to detect the optimum speckle reduction, the intensity detector must resolve the laser illuminated area into resolution spots having a resolution spot size which is greater than or proximately equal to a phase cell size. In other words, in order for the intensity detector to detect the optimum speckle reduction, the intensity detector must be no closer than a distance where the intensity detector resolves the resolution spots into the resolution spot size corresponding to the phase cell size. If the intensity detector is closer than the distance where the intensity detector resolves the resolution spots with the resolution spot size smaller than the phase cell size, the intensity detector will detect a speckle reduction but not the optimum speckle reduction.
The apparatus for reducing speckle includes illumination optics, a diffuser, and projection optics. The illumination optics couple a laser illumination to the diffuser, which is located in a first image plane. The diffuser divides the laser illumination into the phase cells and subdivides the phase cells into the cell partitions. The diffuser also applies the temporal phase variation to the cell partitions. The projection optics project an image of the first image plane onto a diffuse surface and, thus, image the phase cells and the cell partitions onto the diffuse surface. Provided that the temporal phase variation is applied within the integration time of the intensity detector viewing the diffuse surface, the intensity detector detects reduced speckle.
The display apparatus of the present invention incorporates the apparatus for reducing speckle. The display apparatus produces a laser illumination display image comprising pixels at the diffuser and the projection optics project the laser illumination display image onto a display screen. The diffuser divides the pixels into sub-pixels and applies the temporal phase variation to the sub-pixels, which reduces speckle in the laser illumination display image on the display screen.
The diffuser of the present invention includes first and second diffuser cells. Each of the first and second diffuser cell includes first and second diffuser cell partitions. In use, the first diffuser cell partitions induce a first relative phase of zero while the second diffuser cell partitions induce a second relative phase of pi radians. The first and second diffuser cell partitions of the first diffuser cells are preferably arranged in a first Hadamard matrix pattern. The first and second diffuser cell partitions of the second diffuser cell are preferably arranged in a second Hadamard matrix pattern. The first and second Hadamard matrix patterns correspond to first and second Hadamard matrices which satisfy a decorrelation condition.